Fatigue Analysis W- CII

Basics required for performing fatigue analysis: Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. Continued cycling of high stress concentrations may eventually cause a crack which propagates and results in leakages. This failure mechanism is called fatigue. Damage once done during the fatigue process is cumulative and normally unrecoverable. Fatigue can be grouped in two classes; igh cycle fatigue and low cycle fatigue. igh cycle fatigue involves little or no plastic action. Therefore! it is stress"governed. #ormally! a fatigue curve $also called the %&# curve' is generated for every material by e(perimental tests which correlates applied stress with the number of cycles to cause failure. For high"cycle fatigue! the analysis is performed to determine the endurance limit! which is actually a stress level that can be applied for an infinite number  of times without showing any failure. )s a general rule no of cycles *+,- is considered as demarcation point for high and low cycle fatigue. The loading cycles applied in piping design are normally very few in the order of a few thousands. This type of fatigue is identified as low"cycle fatigue. For low"cycle fatigue! the applied stress normally e(ceeds the yield strength of the material! which causes plastic instability in the specimen under test. ut when strain str ain is use used d as the controll controlled ed var variab iable! le! the re resul sults ts in low low"cy "cycle cle region region are reliable reliable as we well ll as reproducible. Sources of Fatigue: For /iping system! Cyclic loadings are primarily due to0 •

Thermal 1(pansion 2 Contraction

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3ibration due to 4ccasional loading

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/ressure variation within /iping system

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5otion wave.

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Due to Flow induced 3ibration

The fatigue process is divided into three stages0 crack initiation from the continued cycling of high stress concentrations! crack propagation to critical size! and unstable rupture of the section. Factors Affecting the Fatigue Behavior: The factors which affect the fatigue behaviour are listed be low0 •

Type Ty pe and #ature of 6oading.

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%ize of Component and stress or strain Distribution.

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%urface finish and Directional /roperties.

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%tress or %train Concentration.

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5ean stress or %train.

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1nvironmental 1ffects.

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5etallurgical Factors and 5aterial /roperties.

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%train 7ate and Fre8uency 1ffects.

Characteristics of Low Cycle Fatigue: •

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Characterized by high loads and a small no. of cycles before failure. ere failure occurs only with stress levels in the plastic range! i.e. significant plastic strain occurs during each cycle.

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The stresses which cause fatigue failure in the piping are the peak stresses.

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9n piping design! most of the loading cycles encountered would be of the low cycle type

Characteristics of High Cycle Fatigue: •

Characterized by high no. of cycles $/referable #:*+,' with relatively low stress levels and the deformation is in elastic range.

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This type of fatigue failure used in the design of rotating machinery.

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This type of fatigue results from strain cycles in the elastic range.

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 ) stress level! endurance limit! may be applied an infinite times without failure! is calculated.

Failure Criteria: